A rectifier is a two terminal device that is commonly used in electric circuits to conduct current in one direction and block current in the opposite direction. The main element of a rectifier is a potential barrier that controls current carrier flow depending on the sign of the potential applied to the external electrodes. Until recently only two major technologies were used to make rectifiers. In Schottky Barrier Diodes (SBD's), the potential barrier is created at the interface between a metal and a semiconductor. Such a barrier is defined by the difference between the work functions of the metal and the semiconductor that make the contact. SBD's provide very good low forward voltage drop (up to 0.4V), which is the major performance characteristic of a diode, but are known to have reliability problems. Due to the lack of carrier modulation they cannot withstand high forward current surges. Additional reliability problem arise due to the spiking during metallization process, which reduces the breakdown voltage and reduces overall yield. Even with the trench Schottky technology, which allows obtain higher breakdown voltage, practical SBD's are limited to breakdown voltages below 250V. The PN-junction technology is typically used for higher voltages. They provide usually higher VF (above 0.7V) and thus lower efficiency, but higher reliability. However, due to carrier density modulation they can withstand large current surges. Also since the maximum electric field is at the PN junction and not at the surface as in a SBD, the metallization spikes do not cause the early breakdown problem.
Other approaches, based on the field effect under an MOS gate, have been proposed in order to combine the high efficiency of a SBD with the high reliability of PN junction diodes. For example, in Pseudo-Schottky Barrier diodes and super barrier rectifiers, the potential barrier is created in the bulk of the semiconductor under the gate via processing (e.g. implantation, diffusion, oxidation etc.). The channel under the MOS gate is only weakly inverted and can be viewed as a barrier for majority carriers. The height of this barrier can be controlled by the gate thickness and the doping concentration under the gate. The presence of the barrier results in rectifying behavior similar to the SBD. SBD's can have a fixed barrier height, corresponding to the metals that make good contact with silicon, while in other prior art devices, the barrier height can be continuously changed. Short channel length and good control of the doping in the channel region are essential to making practical devices. The low voltage (breakdown below 100V) super barrier rectifiers have been shown to combine high reliability (similar to PN-junction diodes) and high efficiency.
However, many high voltage versions of such prior art devices (rated above 150V) exhibit negative differential resistance. Any negative resistance region can be useful to make oscillators, but in rectifiers this is undesirable behavior and needs to be avoided. Thus these prior art devices suffer from significant limitations at high voltages.
To overcome the inability of the prior art to operate reliably at high voltages, it is important to control the negative resistance region, which can involve either an increase or a decrease, depending upon other factors. The source of the negative resistance is the rapid reduction of the drift region resistivity due to the injected carriers. As shown in FIG. 1, which depicts a model of a typical prior art field effect barrier rectifier, the total drift region resistance is typically modeled as being divided into two parts, R1 and R2. The top resistance, R1, typically controls the voltage on the P-N junction, and bottom R2. Once the sum of voltage drops on the resistor R1 and the channel is above the knee voltage V* of the P-N junction, the holes can be injected from P-N junction to the drift region. To maintain quasineutrality the electrons are injected from the substrate. This rapidly growing carrier concentration reduces the resistivity of the drift region and the voltage drop on resistor R2. This voltage drop on the drift region can lead to the negative resistance. The negative resistance can be effectively controlled by varying resistor R1 because it changes the critical current when the injection starts (I*), and because the slope of negative resistance depends on the ratio of R2/R1. Thus the R1 reduction increases the negative resistance region and the R1 increase reduces the negative resistance region.
                    R        2                    R        1              =                            N                      D            ⁢                                                  ⁢            1                          ⁢                  A          1                ⁢                  W          2                                      N                      D            ⁢                                                  ⁢            2                          ⁢                  A          2                ⁢                  W          1                      ,
where A2 is the total area of the drain region and A1 is smaller since current cannot flow through the P region. W1 is close to the thickness of the P region and W2 is the distance between the P region and substrate. The required breakdown voltage sets the donor concentration in the bottom epitaxial region (ND2), but the donor concentration in the top region (ND1) can be adjusted.
One of the ways to control negative resistance in Field Effect Rectifiers is to adjust the donor concentration in the top layer, which was analyzed in Rodov V., Ankoudinov A. L., Ghosh P., Solid State Electronics 2007; 51:714-718. There a reduction of ND1 twice, by the use of a double layer epitaxial structure, was enough to remove negative resistance from the I-V curve. However, this solution of the negative resistance problem may be not the best practical approach, since it is more difficult to manufacture double layer epitaxial structures.
Another major concern is how fast the diode can be switched from forward current conduction to reverse current blocking. One of the major concerns in reverse recovery is the storage time which depends at least in part on how much charge is present in the barrier region. It takes some time to remove this charge, before the depletion layer can be developed to support reverse voltage. The total stored charge still largely determines the total reverse recovery, however some reasonable amount of storage charge is useful since it can provide soft recovery and reduce electro-magnetic interference problems. Thus the softness of reverse recovery is affected by the total stored charge and junction capacitance. To optimize diode reverse recovery it is helpful to be able to quickly deplete the channel region and to be able to trade off between speed of reverse recovery and electromagnetic emissions.
A brief overview of the prior art leads to following conclusions:
Field Effect Diodes provide a good combination of performance and reliability which cannot be achieved by conventional Schottky or PN-junction technologies.
To avoid negative resistance, prior art Field Effect Diodes typically need special means to adjust the top layer resistance.
The ability to rapidly deplete the channel region and operate at high frequency without large electromagnetic interference is desirable in at least some embodiments.